Thick α-Ga2O3 Layers on Sapphire Substrates Grown by Halide Epitaxy

Semiconductors - This work reports on the epitaxial-film growth and characterization of a new wide-gap semiconductor α-Ga2O3. Layers are deposited by chloride vapor phase epitaxy on sapphire...     

Structural properties of β-Ga2O3 formed by dry thermal oxidization process on GaN

Dry thermal oxidation of GaN thin films grown on Al₂O₃ (0001) has been performed at different temperatures. The oxidized samples were investigated through X-ray diffraction (XRD) and atomic force microscope (AFM). For samples oxidized at temperatures from 800 °C to 950 °C, XRD peaks from the (−201), (−402) and (−603) planes of β-Ga₂O₃ were observed, indicating that a β-Ga₂O₃ layer was formed on GaN epitaxially. The epitaxial relationships were determined to be β-Ga₂O₃(−201)||GaN(002) and an in-plane orientation of β-Ga₂O₃[010]||GaN[110]. When the oxidation temperature is increased further to 1000 °C, in addition to the peaks from the (−201), (−402) and (−603) planes, extra peaks corresponding to other planes appeared, indicating that the oxidized layer had deteriorated to polycrystalline Ga₂O_3.     

Structural and Electrical Behaviors of Ag Films Deposited on Liquid Substrates

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High Purity and Large-scale Preparation of β-Ga2O3 Nanowires and Nanosheets by CVD and Their Raman and Photoluminescence Characteristics

Ga2O3 nano-structures, nanowires and nanosheets are produced on Au pre coated（111） silicon substrates with chemical vapor deposition（CVD） technique. By evaporating pure Ga powder in the H2O atmosphere under ambient pressure the large-scale preparation of β-Ga2O3 with monoclinic crystalline structure is achieved. The crystalline structures and morphologies of produced Ga2O3 nano-structures are characterized by means of scanning electron microscope（SEM）, X-ray diffraction（XRD）, selected area electron diffraction （SAED） and transmission electron microscope（TEM）. Raman spectrum reveals the typical vibration modes of Ga2O3 The vibration mode shifts corresponding to Ga2O3 nano-structures are not found. Two distinguish photoluminescence（PL） emissions are found at about 399 nm and 469 nm owing to the VO-VGa excitation and VO-VGaO excitation, respectively. The growth mechanisms of Ga2O3 nanowires and nanosheets are discussed with vapor liquid-solid（VLS） and vapor-solid（VS） mechanisms.     

A review of β-Ga2O3 single crystal defects,their effects on device performance and their formation mechanism

As a wide-bandgap semiconductor (WBG), β-Ga₂O₃ is expected to be applied to power electronics and solar blind UV photodetectors. In this review, defects in β-Ga₂O₃ single crystals were summarized, including dislocations, voids, twin, and small defects. Their effects on device performance were discussed. Dislocations and their surrounding regions can act as paths for the leakage current of SBD in single crystals. However, not all voids lead to leakage current. There’s no strong evidence yet to show small defects affect the electrical properties. Doping impurity was definitely irrelated to the leakage current. Finally, the formation mechanism of the defects was analyzed. Most small defects were induced by mechanical damages. The screw dislocation originated from a subgrain boundary. The edge dislocation lying on a plane slightly tilted towards the (102) plane, the (101) being the possible slip plane. The voids defects like hollow nanopipes, PNPs, NSGs and line-shaped grooves may be caused by the condensation of excess oxygen vacancies, penetration of tiny bubbles or local meltback. The nucleation of twin lamellae occurred at the initial stage of " shoulder part” during the crystal growth. These results are helpful in controlling the occurrence of crystal defects and improving the device performance.     

Optical and Electrical Properties of Sensitized Polyaniline by Electrochemical Polymerization

To sensitize polyaniline with dyes by electrochemical polymerization,HClO4 is employed as the dopant and oxidant,and the polyaniline with differnet sensitive properties is synthesized.The effect of sensitized emeraldine salt on the absorption spectrum is discussed in details.The maximum conductivity of sensitized films reaches 1.22S/cm,and investigation on dye sensitizing of the polymer reveals that C.I.Direct Blue 71,C.I.Direct Blue 84,C.I.Direct Black 19 and CuPc-(COOH)4 may enhance the photoconductivity of polyaniline greatly.     

Decarbonization and Decolorization of Large Sapphire Crystals Grown by the Temperature Gradient Technique

温度梯度法(temperature gradient technique,TGT)生长的Al2O3晶体因石墨发热体在高温时的挥发和原料中过渡性金属离子的存在,在不同部位呈现不同颜色,一般上部为浅红色,尾部为浅黄绿色.将TGT法生长的Al2O3晶体(φ110×80mm3)依次经过高温氧化气氛、高温还原气氛脱碳、去色退火实验,即"两步法"退火实验,晶体变成无色、透明.经测试,Al2O3晶体的完整性、光学透过率和光学均匀性均有显著提高.     

Temperature-dependent electrical properties of β-Ga2O3 Schottky barrier diodes on highly doped single-crystal substrates

Beta-phase gallium oxide(β-Ga_2O_3) Schottky barrier diodes were fabricated on highly doped single-crystal substrates,where their temperature-dependent electrical properties were comprehensively investigated by forward and reverse current density – voltage and capacitance – voltage characterization. Both the Schottky barrier height and the ideality factor showed a temperature-dependence behavior, revealing the inhomogeneous nature of the Schottky barrier interface caused by the interfacial defects. With a voltage-dependent Schottky barrier incorporated into thermionic emission theory, the inhomogeneous barrier model can be further examined. Furthermore, the reverse leakage current was found to be dominated by the bulk leakage currents due to the good material and surface quality. Leakage current per distance was also obtained. These results can serve as important references for designing efficient β-Ga_2O_3 electronic and optoelectronic devices on highly doped substrates or epitaxial layers.     

Effect of Surface-Covered Annealing on the Optical Properties of ZnO Films Grown by MOCVD

研究了暴露在空气中退火和表面覆盖蓝宝石基板退火对MOCVD生长的ZnO薄膜光学性质的影响. 研究发现，暴露在空气中退火虽可以去除薄膜中的氢杂质，并在低温光致发光(PL)谱中观察到与氢相关的束缚激子峰消失，但是退火后样品室温PL谱中可观察到很强的可见光发射，表明样品中引入了大量的深能级，样品的自由激子发光没有增强. 而表面覆盖蓝宝石基板退火的样品，有效去除了氢杂质，但没有观察到可见光发射，说明表面覆盖蓝宝石基板退火可以有效地保护ZnO表面不分解，不生成深能级中心. 由于激子束缚中心的减少，表面覆盖退火样品的自由激子发射大大增强.     

Characterization of GaN Buffer Layers and Its Epitaxial Layers Grown by MOCVD

Low-pressure MOCVD has been used to investigate the properties of low-temperature buffer layer depodition conditions and their influence on the properties of high-temperature GaN epilayers grown subsequently.It is found that the surface morphology of the as-grown buffer layer after thermal annealing at 1030℃and 1050℃ depends strongly on the thickness of the buffer layer.In particular when a thick buffer layer is used, large trapezoidal nuclei are formed after annealing.     

Effect of Ultraviolet Radiation and Electric Field on the Conductivity of Structures Based on α- and ε-Ga2O3

Semiconductors - The effect of ultraviolet radiation and a strong electric field on the conductivity of structures based on two types of polymorphic gallium-oxide films is studied. Both types of...     

Structural and optical properties of Zn-doped β-Ga_2O_3 films

正Intrinsicβ-Ga_2O_3 and Zn-dopedβ-Ga_2O_3 films were prepared using RF magnetron sputtering.The effects of the Zn doping and thermal annealing on the structural and optical properties are investigated.In comparison with the intrinsicβ-Ga_2O_3 films,the microstructure,optical transmittance,optical absorption,optical energy gap,and photoluminescence of Zn-dopedβ-Ga_2O_3 films change significantly.The post-annealedβ-Ga_2O_3 films are polycrystalline.After Zn doping,the crystallization deteriorates,the optical band gap shrinks,the transmittance decreases and the UV,blue,and green emission bands are enhanced.     

High Performance β-Ga2O3 Schottky Barrier Transistors with Large Work Function TMD Gate of NbS2 and TaS2

Gallium trioxide, β-Ga₂O₃, has been recently studied due to its promising semiconducting properties as active material in transistors or Schottky diodes. Transistors with β-Ga₂O₃ channels are mostly metal oxide field effect transistors (MOSFET), and they show very negative threshold voltages (V_th) in general. Metal semiconductor field effect transistors (MESFETs) with top gate are also reported with less negative V_th. Still, β-Ga₂O₃ MESFETs are only a few. Here, bottom gate architecture β-Ga₂O₃ MESFETs using transition metal dichalcogenide (TMD) NbS₂ and TaS₂ are reported. Due to the large work functions of those metallic TMDs, the MESFETs display minimum subthreshold swing of 61 mV dec⁻¹, small V_th of −1.2 V, minimum OFF I_D of ≈100 fA, and maximum ON/OFF current ratio of ≈10⁸. Both β-Ga₂O₃ Schottky diodes with TaS₂ and NbS₂ display good junction stability even after 300 °C measurements in 10 mTorr vacuum. When the β-Ga₂O₃ MESFET with TaS₂ gate is integrated as a switching FET into an organic light emitting diode (OLED) circuit, it demonstrates long-term leakage endurance performance, maintaining an OLED brightness higher than 58% of the initial intensity after 100 s passes since the ON-switching point, which is even superior to the performance of conventional a-IGZO MOSFET switch.     

X-Band GaN Power HEMTs with Power Density of 2.23W/mm Grown on Sapphire by MOCVD

The growth,fabrication,and characterization of 0.2μm gate-length AlGaN/GaN HEMTs, with a high mobility GaN thin layer as a channel,grown on (0001) sapphire substrates by MOCVD,are described.The unintentionally doped 2.5μm thick GaN epilayers grown with the same conditions as the GaN channel have a room temperature electron mobility of 741cm2/(V·s) at an electron concentration of 1.52e16cm-3.The resistivity of the thick GaN buffer layer is greater than 1e8Ω·cm at room temperature.The 50mm HEMT wafers grown on sapphire substrates show an average sheet resistance of 440.9Ω/□ with uniformity better than 96%.Devices of 0.2μm×40μm gate periphery exhibit a maximum extrinsic transconductance of 250mS/mm and a current gain cutoff frequency of 77GHz.The AlGaN/GaN HEMTs with 0.8mm gate width display a total output power of 1.78W (2.23W/mm) and a linear gain of 13.3dB at 8GHz.The power devices also show a saturated current density as high as 1.07A/mm at a gate bias of 0.5V.     

Evolution of Structural,Optical and Electrical Characteristics of Spin-Coated CdO Thin Films with the Gelation State of the Sol–Gel

The present work is intended to investigate the influence of the gelation state of the sol–gel on the properties of spin-coated cadmium oxide (CdO) thin films. The viscosity of the sol–gel increases at a slow uniform rate up to 5 days (break-off point) after which the rise becomes progressively more rapid and it attains saturation after 10 days of gelation. Films have been grown with gelation times of 2 days, 4 days, 6 days, 8 days, 9 days and 10 days. The visual characteristics of the films have been discussed in terms of the centrifugal force acting on the sol–gel. The sol–gel viscosity seems to be better suited to represent the gelation state of the sol–gel rather than the gelation time. The x-ray diffraction studies show that lower gelation times and lower sol–gel viscosities give rise to single crystalline CdO thin films while gelation times of 6 days and above (i.e. sol–gel viscosities of 2.92 × 10^?3 N s m^?2 and more) yield polycrystalline CdO thin films. The gelation state of the sol–gel has been found to have a strong bearing on the properties of CdO thin films, and highly conducting and transparent CdO thin films can be achieved by controlling the gelation state of the sol–gel and the results obtained have been reported.     

N Doping and Al-N Co-doping in Sol–Gel ZnO Films: Studies of Their Structural,Electrical, Optical,and Photoconductive Properties

We have studied the structural, electrical, optical, and optoelectronic properties of N-doped and Al-N co-doped ZnO films grown by sol–gel technique. Both the undoped and doped films have a hexagonal wurtzite-type structure. The undoped film shows a very low electron concentration. The N-doped film exhibits an anomalous conduction type, while the Al-N co-doped film shows relatively stable p-type conduction. Though the transparency in the visible region is retained in the doped films, its optical qualities are degraded as indicated by photoluminescence results. The N-doped film has better response to ultraviolet light compared with the undoped and co-doped films. These results are important for the growth and development of p-type ZnO films using the very low-cost sol–gel method.     

Optical and Electrical Properties of Polyimide Thin Films Doped-Cu-phthalocyanine Polymerized by Vapor Phase Deposition

Using Cu-phthalocyanine(CuPc),4,4’-diaminodiphenyl ether and pyromellitic dianhydride as monomer materials, polyimide(PI) thin films doped-CuPc have been prepared onto glass substrate by vapor phase co-deposition polymerization under a vacuum of 2×10~(-3)Pa and thermal curing of polyamic acid film in at temperature of 150-200℃ for 60min. In this process, the polymerization can be carried out through controlling the stoichiometric ratio, heating time and deposition rates of the three monomers. IR spectrum identifies the designed chemical structure of the polymer. The absorption of polyimide doped-CuPc is very intense in vis-range and near-infrared by UV-Vis spectrum. And, the PI films doped-CuPc polymerized by vapor phase deposition have uniformity, fine thermal stability and good nonlinear optical properties, and the third-order optical nonlinear susceptibility χ~((3)) with degenerate four-wave mixing can be 1.984×10~(-9)ESU.     

Structural Characterization and Magnetic and Electrical Properties of La0.7Ca0.3MnO3–La1.5Sr0.5NiO4 Nanocomposites

Nanocomposite samples of (1 ? x)La_0.7Ca_0.3MnO₃ + xLa_1.5Sr_0.5NiO₄ (x = 0 to 0.3) were synthesized by a combination of the mechanical milling and solid-state reaction methods. X-ray diffraction analyses and magnetic measurements indicated that no reaction occurred between La_0.7Ca_0.3MnO₃ (LCMO) and La_1.5Sr_0.5NiO₄ (LSNO). The Curie temperature (T _C) was almost independent of x, while the metal–insulator transition temperature (T _MI) shifted from 251 K for x = 0.0 to 65 K for x = 0.2. The samples with x ≥ 0.25 exhibited insulating behavior in the temperature range from 30 K to 300 K. Addition of LSNO substantially increased the resistivity of the composites. This is attributed to enhanced magnetic disorder at LCMO grain boundaries due to the addition of LSNO. The temperature dependence of the resistivity, ρ(T), could be described by the phenomenological percolation model of phase segregation. Fitting the experimental ρ(T) data in the temperature range of 30 K to 300 K indicated that the activation energy of the composites increases as a function of the LSNO doping concentration (x).